FIG. 1 shows a prior art centrifugal blower wheel assembly 10 which includes a backplate 12, a hub 14, and a plurality of blades 16. The hub 14 and the blades 16 are attached to the backplate 12, which is typically a separate part. The blades 16 are secured to a ring 17; alternatively, the blades may be formed as a single piece, known as a bladestrip. The assembly 10 is used by attaching it to a rotational mechanism (not shown) via the hub 14 by means of a shaft (not shown). Rotation of the shaft causes rotation of the hub 14, backplate 12 and blades 16, thereby providing air flow. The connection between the backplate 12 and the hub 14 therefore is required to transmit the rotational torque of the shaft.
The maximum torque the hub 14 can withstand before coming loose with respect to the backplate 12 is termed the holding torque. The holding torque is a function of the way in which the hub is attached to the backplate. In addition, the holding torque can decrease over time as use changes the strength of that attachment. If the holding torque is exceeded, the hub becomes loose and will spin independently of the backplate 12, resulting in a catastrophic failure of the blower wheel assembly.
FIGS. 2A-2C illustrate details of a prior art hub and backplate configuration. The hub 14 has a concentric rim or lip 1 8 protruding from a front surface 19 of the hub 14. The lip 18 is designed to be placed in a hole 20 of the backplate 12 as illustrated in FIG. 2C. The hub 14 has a back surface 22 through which a hole 24 extends in order to receive a shaft (not shown) or other member for rotation. A threaded set screw hole 26 is provided along a radius of the hub. A set screw (not shown) can be threaded in the hole 26 to allow for the assembly 10 to be fixed with respect to the shaft within the hole 24.
The hub 14 is attached to the backplate 12 by forcing back (via stamping, for example) the rim or lip 18 while the rim or lip 18 extends through the hole 20 of the backplate 12, thereby crimping the rim or lip 18 against the backplate 12 and holding the hub 14 thereto. In some circumstances, however, the holding torque for this type of arrangement is either insufficient or inconsistent, and therefore undesirable.
The backplate 12, the hub 14, and the blades 16 are all typically made of steel, which provides for high strength, low cost, and ease of manufacture.
An objective of the invention is to provide a blower wheel assembly with a hub that is more strongly attached to the backplate, that can be used over a wide range of temperatures, and that is inexpensive to manufacture.